Marie-
I've never heard of the term, "waterline" until I got to this forum-even in my classes, we never used that term.
It's the area of the lid right under the top lash line. Darkening that area marries your lashes together and makes them look fuller without looking like you have a lot of makeup on. I like to use either Bobbi Brown's gel eyeliner or (much easier) Urban Decay's 24/7 liner. The gel products are not tested to go in the eye area, but it has lasting stay power. The Urban Decay liner has become my new favorite liner because it's extremely soft, but has lasting power as well.
As far as hot weather, we have talent in makeup for hours under hot lights, so this is what I do-
I always use Loreal's Voluminous in the waterproof formula (regardless if it's hot).
Urban Decay Eyeshadow Primer with dry shadow -or-
Apply eyeshadows wet with La Femme's Cake Eyeliner Sealer (bit tricky to blend and doesn't work well with matte shadows) -or-
Use Mac cream Paint, Bobbi Brown cream shadows or Bourjois cream shadows-these can be tricky too, because once they dry, you can't move them. Bourjois has a very limited range of colors that are all super shimmery, but it's waterproof. Sometimes I use one of these cream shadows as a base to apply powder shadow over it.
Use a cream blush, apply foundation, then brush a powder blush over. THEN set whole face with face powder with a puff (more than what you would normally use, push the puff into the foundation), dust off the excess with a brush, then spray Evian to absorb the powder. For extra staying power, we sometimes use Reel Creations Blue Aqua Sealer (which waterproofs the makeup).
I have seen a lot of positive reviews for Estee Lauder's Double Wear for it's durability and non-transfer abilities. You might want to check that out too.
Geez, I love makeup!
Cheers!